A number of problems exist in previous condenser fan control systems.
Instead of controlling condenser fans to maintain an optimum system pressure differential between the high and low pressure sides of a refrigeration system, previous systems controlled condenser fans in response to discrete predetermined, and readily identifiable system conditions. More specifically, the previous systems were event orientated. For instance, condenser fans were turned on or off as the compressor load step was increased or decreased. Similarly, condenser fans were turned on or off as outside air temperature exceeded or dropped below specific, predetermined levels. This event orientated approach compromises efficiency from a condenser fan staging point of view because such an approach does not react to system requirements unless a specific event occurs.
Previous systems responded to known criteria including outside air temperature, compressor load step and sometimes in accordance with a condenser pressure switch. Variable capacity compressors have changed this by introducing an unknown independent variable, the compressor capacity. The compressor capacity at any given time is usually unknown, is not easily determinable, and is capable of independent variance in response to system conditions. Attempts have been made to compensate for the introduction of variable capacity compressors into refrigeration systems by replacing a multiplicity of single speed condenser fans with a single larger variable speed fan. However, this requires an expensive variable speed fan, a complex control strategy, a variable speed fan controller and feedback from the variable capacity compressor to the variable speed fan controller indicating the present compressor capacity. Furthermore, the additional component requirements and controls add considerable expense and complexity to newly manufactured systems without addressing the needs of existing systems which may only intend to upgrade the compressor.
Additionally, previous systems have not taken any steps to prevent a system safety device, such as a high pressure cutout, from tripping. The tripping of a system cutout shuts down the refrigeration system until the refrigeration system is restarted.
Finally, previous condenser fan control systems did not respond well to low ambient temperatures. The operation of air cooled condensers during low ambient temperature conditions requires that the condenser airflow be reduced progressively as the ambient temperature decreases so that the condenser pressure and temperature are maintained at the level necessary for proper system operation. At low ambient temperature, previous systems were faced with the dilemma of either turning all fans off, or leaving too many fans on. Either choice compromised efficiency.